/// Origin of the argument's value
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
#[non_exhaustive]
pub enum ValueSource {
    /// Value came [`Arg::default_value`][crate::Arg::default_value]
    DefaultValue,
    /// Value came [`Arg::env`][crate::Arg::env]
    EnvVariable,
    /// Value was passed in on the command-line
    CommandLine,
}